Hardware - Soapbox Derby Cart Speedometer
Using an Atmel ATTiny2313 microcontroller and an HD44780-compatible character LCD display, along with a reed switch and some magnets, I was able to construct a tiny speedometer/trip computer for a soapbox derby cart.It's primary function was to measure the top speed of the cart, total distance travelled, and total race time.
It measured the speed of the cart using a permanent magnet attached to the rear wheel's rim, and a stationary reed switch attached to the soapbox derby cart. Every time the wheel revolved, the magnet would pass by the reed switch, thereby momentarily opening it. The microcontroller calculated the speed of the cart simply by dividing the circumference of the rear wheel by the period of the wheel's revolution.